The number of connections on integrated circuits is becoming ever greater, while said integrated circuits are at the same time becoming miniaturized to an ever greater extent. The difficulties expected in the course of this increase in miniaturization in the application of solder paste and component mounting are intended to be overcome by new housing forms, where emphasis may be given here in particular to single-chip, few-chip or multi-chip modules in the ball grid array package (DE-Z productronic 5, 1994, pages 54, 55). These modules are based on a plated-through substrate, on which contact is made with the chips for example via bonding wires or by means of flip-chip mounting. The ball grid array (BGA) is situated on the underside of the substrate, and is often also referred to as a solder grid array or solder bump array. The ball grid array comprises solder studs arranged over the area on the underside of the substrate, which allow surface mounting on the printed circuit boards or assemblies. The arrangement of the solder studs over an area allows large numbers of connections to be realized, in a coarse grid of 1.27 mm, for example.
In the case of so-called MID technology (MID =Molded Interconnection Devices), injection-molded parts with integrated conductor runs are used instead of conventional printed circuits. High-quality thermoplastics suitable for the injection molding of three-dimensional substrates form the basis of this technology. By comparison with conventional substrate materials for printed circuits, such thermoplastics are distinguished by better mechanical, chemical, electrical and environmental properties. In the case of one specific direction of MID technology, so-called SIL technology wherein SIL stands for the German phrase of Sitzgiexcex2teile mit integrierten Leiterzxc3xcgen which phrase translates to injection-molded parts with integrated conductor runs a metal layer applied to the injection-molded parts is structured without using the mask technique that is otherwise customary, by means of a special laser structuring method. In this case, a plurality of mechanical and electrical functions can be integrated in the three-dimensional injection-molded parts with structured metalization layer. The housing support function is simultaneously taken over by guides and snap-connections while the metalization layer, in addition to the wiring and connection function, also serves as electromagnetic shielding and provides for good heat dissipation. In order to produce electrically conductive. cross connections between two wiring installations on mutually opposite surfaces of the injection-molded parts, corresponding plated-through holes are produced as early as during the injection-molding process. The inner walls of these plated-through holes are then likewise coated with a metal layer during the metalization of the injection-molded parts. Further details relating to the production of three-dimensional injection-molded parts with integrated conductor runs are revealed for example in DE-A-37 32 249 or EP-A-0 361 192.
According to a variant of MID technology which is disclosed in EP-A-0 645 953, a substrate is produced by injection molding and provided with a well and then a first conductor plane, a dielectric layer and a second conductor plane are successively produced, whereupon an electronic component is incorporated in the well, the connections of the component are electrically conductively connected to assigned connection areas on the substrate, preferably by bonding, and then an encapsulation is formed for the component by the well being filled with plastic. A compact, thin structure with a high wiring density is produced. The recessed mounting and encapsulation of components in wells of the injection-molded substrate not only results in a thickness reduction but also attains optimum protection of the component and the connection wiring thereof.
WO-A-96 096 46 has disclosed a so-called polymer stud grid array (PSGA), which combines the advantages of a ball grid array (BGA) with the advantages of MID technology. The new design was called a polymer stud grid array (PSGA) on the basis of the ball grid array (BGA), the term xe2x80x9cpolymer studxe2x80x9d being intended to refer to polymer studs which are concomitantly formed during the injection molding of the substrate. The new design is suitable for single-chip, few-chip or multi-chip modules and comprises
an injection-molded three-dimensional substrate made of an electrically insulating polymer,
polymer studs which are arranged over the area on the underside of the substrate and are concomitantly formed during the injection-molding process,
external connections which are formed on the polymer studs by a solderable end surface,
conductor runs which are formed at least on the underside of the substrate and connect the external connections to internal connections, and
at least one chip which is arranged on the substrate and whose connections are electrically conductively connected to the internal connections.
In addition to the simple and cost-effective production of the polymer studs during the injection molding of the substrate, the production of the external connections on the polymer studs can also be carried out with minimal effort together with the production of the conductor runs, which is normal for MID technology and SIL technology. The fine laser structuring which is preferred in SIL technology enables the external connections to be realized on the polymer studs with large numbers of connections in a very fine grid. It should be emphasized, furthermore, that the thermal expansion of the polymer studs corresponds to the thermal expansions of the substrate and to that of the printed circuit board accommodating the module. Should any mechanical stresses occur, then the elastic properties of the polymer studs allow at least partial compensation. The dimensional stability of the external connections formed on the polymer studs also allows the reliability for repair and replacement to be considerably enhanced by comparison with ball grid arrays, whose connections are formed by solder studs. In the case of the polymer stud grid array, the polymer studs and the chip or the chips are usually arranged on the same side of the substrate. In the case of a substrate provided with plated-through holes, the polymer studs and the chip or the chips may perfectly well be arranged on different sides of the substrate as well. Such an arrangement of polymer studs and chips on opposite sides of the substrate is of interest particularly for large chips, which require a multiplicity of assigned external connections.
WO-A-89 00346 discloses single-chip modules which are suitable for surface mounting and are based on an injection-molded three-dimensional substrate with plated-through holes. In addition to these plated-through holes, during the injection-molding process, the substrate acquires a well, arranged centrally on the top side, and a multiplicity of polymer studs arranged in one or else in two peripheral rows on the underside. The chip arranged in the well on the top side is connected to assigned conductor tracks, which lead to the outside in the form of strips, via fine contact-making wires. These conductor tracks are then electrically conductively connected to the assigned, surface-metalized polymer studs over plated-through holes arranged in the outer region. If the edge regions of the substrate are then separated with cutting lines which pass centrally through the plated-through holes, then electrically conductive cross connections with a semicircular cross section are produced, which electrically conductively connect the outer ends of the conductor tracks arranged on the surface of the substrate to the assigned polymer studs arranged on the underside of the substrate.
R. C. ANDERSON et al.: xe2x80x9cPrinted circuit boards with multi-conductor through-holesxe2x80x9d, TECHNICCAL DIGESTxe2x80x94WESTERN ELECTRIC, No. 41, January 1976, pages 1-2, discloses substrates with plated-through holes which have a roof-shaped or convex contour. During the photolithographic production of the conductor tracks on the top side and underside of the substrate, optionally a plurality of conductor track-type cross connections are also produced in the same work operation in the region of the plated-through holes, which cross connections connect together mutually assigned conductor tracks on the top side and underside of the substrate.
The invention of providing a molded substrate with at least one edge with an oblique, roof-shaped or convex contour so that a metallized layer can be partially removed to form a conductive structure extending from one side of the substrate to the other is based on the problem, in the context of MID technology, of simplifying the production of electrically conductive cross connections between conductor structures on the top side and underside of a substrate. In this case, the cross connections are intended to be suitable, in particular, also for the polymer stud grid arrays explained above.
The invention is based on the insight that cross connections can be produced at the same time as the laser structuring of the conductor structures provided that the cross connections are produced, on at least one end of the substrate, at edges with an oblique, roof-shaped or convex contour. The structuring of the cross connections according to the invention can be carried out with minimal outlay since no additional work operation is necessary.
Advantageous refinements of the method according to the invention are molding the substrate by injection molding, compression molding or transfer molding The refinement of the method enables the substrate to be produced simply and in a manner suitable for mass production, in particular, by means of injection molding.
In contrast to a convex contour, the contours of either a roof-shape or oblique facilitate the laser structuring of the cross connections by virtue of their pronounced edges. An accordance with claims 4 and 11, effective laser structuring of the cross connections is ensured in this case. through angles of inclination of at least 110xc2x0 with respect to the horizontal and/or at least 20xc2x0 with respect to the vertical.
The refinement of molding studs on the underside of the substrate, which studs are provided with a solderable end surface after the metallization step; enables the cross connections according to the invention to be realized in polymer stud grid arrays.
The development of applying the metallized layer by electroless and electrodeposition of copper; has the advantage that, for the application of the metallization layer, it is possible to resort to technologies which have long proven their worth in the production of printed circuits.
Although the method according to the invention can, in principle, also be carried out using the semiadditive technique, the subtractive technique affords quite a few advantages. In addition to the simple and economical production of the desired conductor patterns, emphasis may be given here in particular to the simple laser structuring of an etching resist. The etching resist can in this case be applied in a particularly simple manner by chemical deposition or electrodeposition of tin or tin-lead.
In accordance with claim 12, the cross connections according to the invention can preferably be used in a polymer stud grid array with a chip arranged on the top side of the substrate. The arrangement of the chip on the top side of the substrate and the arrangement of the polymer studs on the underside of the substrate in this case result in ideal preconditions for realizing so-called chip scale packages, in the case of which the dimensions of the array essentially correspond to the dimensions of the chip.